Yes the Sonicwall does have a static route back to the 192.168.100.0 subnet so it knows where my workstation lives. I have always been able to ping the Sonicwall from my workstation.

I should also have mentioned that performing a ping or tracert on the Mikrotik (WinBox) it also tried to use the 192.168.102.254 (Telstra Cisco) as the default route most of the time, not always, occasionally it also went the via the correct gateway.

But I have fixed the problem, apparently. Last night whilst researching the v4.5 upgrade I noticed the mention of a bug fix for something to do with delete static routes?

When I origninally setup the Mikrotik the default route was directed to 192.168.102.254 which at that stage was the Sonicwall as the Cisco was not connected. Before I connected the Mikrotik to the production network I changed the Soincwall to .253 and EDITED the default route on the Mikrotik.

All I have done to apparently correct the problem is to DELETE the default route completely and ADD it back, no change in the actual settings. Everything now routes as expected?

There is a wider default route problem with the Mikrotik which hopefully only exists at this firmware version and is corrected by v4.5 which we are about to test.

If I disconnect the Cisco from the network the Mikrotik no longer attempts to use the Cisco as its default route gateway. Have to suspect the Mikrotik is listening to route updates from the Cisco (it does have a default route configured) even though all routing protocols are disabled on the Mikrotik?

Now that the Cisco is out of the network we see a second problem - the Mikrotik randomly returns Destination Host Unreachable to outside IP addresses. That is-

Can get to outside public IPs A and B

Few minutes later can get to A but NOT B

Few minutes later can get to B and NOT A

Few minutes later can get to neither A or B

Few minutes later can get to BOTH A and B

This only happens for IPs that a routed via the Default Route static entry, NOT for IPs that are known by specific routes.

We are currently chasing this exact problem on another RB1000U running v3.23. This second RB1000 is only 5 days old, is in acompletely different network, is not routing outbound through a SonicWall it has one interface on a public IP, is routing only no NAT and no firewall configurations, has been set up by another engineer not me, is connected to a different ISP and displays EXACTLY the problem above.